Feat: add IMAP data source integration with configuration and sync capabilities (#12316)

### What problem does this PR solve?
issue:
#12217 [#12313](https://github.com/infiniflow/ragflow/issues/12313)
change:
add IMAP data source integration with configuration and sync
capabilities

### Type of change

- [x] New Feature (non-breaking change which adds functionality)

common/data_source/config.py

@@ -57,6 +57,7 @@ class DocumentSource(str, Enum):
     ASANA = "asana"
     GITHUB = "github"
     GITLAB = "gitlab"
+    IMAP = "imap"
 
     
 class FileOrigin(str, Enum):
@@ -266,6 +267,10 @@ ASANA_CONNECTOR_SIZE_THRESHOLD = int(
     os.environ.get("ASANA_CONNECTOR_SIZE_THRESHOLD", 10 * 1024 * 1024)
 )
 
+IMAP_CONNECTOR_SIZE_THRESHOLD = int(
+    os.environ.get("IMAP_CONNECTOR_SIZE_THRESHOLD", 10 * 1024 * 1024)
+)
+
 _USER_NOT_FOUND = "Unknown Confluence User"
 
 _COMMENT_EXPANSION_FIELDS = ["body.storage.value"]